Platinum group element,mineralisation in the Munni Munni Complex,western Australia

Summary The late Archaean Munni Munni Complex occupies an elliptical area of 9 by 25 km, the southern half of which is covered unconformably by a 2.7 Ga volcanic sequence. The Complex consists of a lower 1850 m thick Ultramafic Series (UMS) and an upper Gabbroic Series (GS) at least 3600 m thick, and is in the form of an elongate funnel. The UMS is made up of macrorhythmic cycles of dunite, wehrlite and clinopyroxenite, while the GS shows an uninterrupted fractionation trend from pigeonite gabbros through pigeonite-magnetite gabbros to granophyres. The base of the GS is very sharp, and marked by simultaneous appearance of cumulus plagioclase and pigeonite. GS cumulates show a monotonous upward increase in Fe/Mg and an absence of cyclic layering, indicating crystallization in a closed chamber.The top of the UMS is a distinctive 30 m thick layer of bronzite-porphyritic orthocumulate websterite, which continues up the side walls as a marginal zone in contact with progressively more fractionated gabbros. A pyroxenite dyke intersects the sloping floor of the intrusion at a level close to the top of the UMS, and appears to have fed the uppermost layers of the UMS.Cu-rich magmatic sulphides are weakly disseminated throughout the porphyritic websterite layer, increasing in abundance to 1–3% in a semi-continuous augite orthocumulate layer a few metres below the gabbro. This layer extends over 8.2 km, averages 2.5 m in thickness, and has an average grade of 2.9 g/t Pt + Pd + Au, 0.2% Ni and 0.3% Cu with local higher grade zones. In about 40% of intersections, peak PGE, Au, Cu and Ni grades are coincident, while in the remainder peak PGE grades are offset about 1–2 m below the peak Cu and Ni grades.Coincident intersections are probably derived by homogenization of original offset intersections. Peak PGE grades become lower and more widely dispersed farther away from the intrusion walls.PGE-enriched sulphides also occur close to the websterite-gabbro contact where the websterite occupies a marginal position on the side wall. The marginal websterite zone and the porphyritic websterite layer are physically contiguous and petrographically similar, and are probably correlative.Microprobe data on cumulus pyroxenes indicate that the porphyritic websterite layer crystallised from a mixture of a relatively Mg- and Cr-rich M magma, parental to the Ultramafic Series, and an Fe-rich, strongly Cr-depleted gabbroic G magma. Pyroxenes from the PGE horizon are very low in Cr, suggesting that they crystallised from a G-rich hybrid.The websterite formed as a result of an influx of dense G magma which mixed with hotter resident M magma. The upper few metres of the websterite, including the PGE-rich sulphides, accumulated during a period of quiescence at the end of the influx phase. The PGE-rich sulphides formed by fractional segregation of sulphide liquid from a 500 to 1000 m thick layer of silicate magma.Munni Munni PGE mineralisation shows some striking similarities to that of the Great Dyke, particularly in the stratigraphic position of the mineralisation, the vertical distribution of PGE through the sulphide layer, and the lateral distribution of grades.

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Platin-Mineralisation im Munni Munni-Komplex, West-Australien

Zusammenfassung Der spät-archaische Munni Munni-Komplex bedeckt eine elliptische Fläche von 9 × 25 km, deren südliche Hälfte diskordant von einer 2.7 Ga alten vulkanischen Abfolge überlagert wird. Der Komplex besteht aus einer unteren, 1850 m mächtigen ultramafischen Serie (UMS) und einer oberen gabbroischen Serie, die mindestens 3600 m mächtig ist und die Form eines länglichen Trichters hat. Die UMS besteht aus makrorhytmischen Zyklen von Dunit, Wehrlit und Klinopyroxenit, während die GS einen ununterbrochenen Fraktionierungs-Trend von Pigeonit-Gabbros über Pigeonit Magnetit-Gabbros zu Granophyren zeigt. Die Basis der GS ist scharf und wird durch das gleichzeitige Erscheinen von Cumulus-Plagioklas und Pigeonit definiert. GS Cumulate zeigen gegen das Hangende zu eine monotone Zunahme von Fe/Mg und ein Fehlen zyklischen Lagenbaues, was auf Kristallisation in einer geschlossenen Kammer hinweist.Der oberste Teil der UMS ist eine deutlich ausgebildete, 30 m mächtige Lage von Bronzit-porphyritischem Orthokumulat-Websterit, welche sich an den Seitenwänden als randliche Zone fortsetzt, die in Kontakt mit zunehmend mehr fraktionierten Gabbros ist. Ein Pyroxenet-Gang durchschlägt den geneigten Boden der Intrusion im Bereich der obersten UMS, und dürfte als Zufuhrkanal für die obersten Lagen der UMS gedient haben.Eine schwache Dissemination von Cu-reichen magmatischen Sulfiden ist im Gesamtbereich der porphyritischen Websterit-Lage zu beobachten; in einer Augit-Orthocumulat-Lage wenige Meter unterhalb des Gabbros steigt diese auf 1–3% Cu-Sulfide an. Diese Lage erstreckt sich über 8.2 km, ist im Durchschnitt 2.5 m mächtig, und hat einen Durchschnittsgehalt von 2.9 g/t Pt + Pd + Au, 0.2% Ni und 0.3% Cu, mit lokal reicheren Zonen. In etwa 40% der untersuchten Bohrkerne fallen maximale Gehalte an PGE, Au, Ni und Cu zusammen, während sonst maximale PGE-Gehalte etwa 1–2 m unterhalb der Cu- und Ni-Maxima auftreten.Zusammenfallende Maxima dürften durch Homogenisation ursprünglich separater Maxima entstanden sein. Mit zunehmender Entfernung von den Rändern der Intrusion nehmen PGE Gehalte ab und werden unregelmäsiger.PGE-reiche Sulfide kommen auch nahe am Websterit-Gabbro-Kontakt vor, wo der Websterit eine randliche Position einnimmt. Die randliche Websterit-Zone und die porphyritische Websterit-Lage hängen zusammen, sind petrographisch ähnlich, und sind wahrscheinlich zu korrellieren.Mikrosonden-Analysen von Kumulus-Pyroxenen zeigen dass die porphyritische Websterit-Lage aus einer Mischung von relativ Mg- und Cr-reichem M-Magma dem die ultramafische Serie zuzuordnen ist, und einem Fe-reichen, Cr-armen gabbroischen G-Magma entstanden ist. Pyroxene aus der PGE-Lage führen sehr niedrige Cr-Gehalte; dies dürfte auf Kristallisation aus einem G-reichen Hybrid-Magma zurück gehen.Der Websterit wurde als das Resultat der Zufuhr von dichtem G-Magma das sich mit höher temperiertem M-Magma mischte, gebildet. Die obersten Meter der Websterit Abfolge, mit den PGE-reichen Sulfiden, bildeten sich während einer ruhigen Periode am Ende der Influx-Phase. Die PGE-reichen Sulfide sind das Produkt fraktionierter Segregation von sulfidischer Schmelze aus einer 500 bis 1000 m mächtigen Lage silikatischen Magmas.Die PGE-Mineralisation des Munni Munni-Komplexes ist der des Great Dyke von Zimbabwe in vieler Hinsicht ähnlich, besonders was die stratigraphische Position, die vertikale Verteilung der PGE in der Sulfid-Lage, und die laterale Verteilung der Gehalte betrifft.

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With 7 Figures and 1 Plate     

Halogen geochemistry of the Great Dyke,Zimbabwe

 Apatite from the Great Dyke of Zimbabwe is relatively rich in the hydroxy-fluorapatite end-members. The mole fraction of fluorapatite increases from approximately 40% in cumulates of the Ultramafic Sequence to over 60% in a sample near the top of the exposed Mafic Sequence. The chlorapatite component decreases from a typical high of 10–20 mole% in the Ultramafic Sequence to about 1% in the uppermost part of the Mafic sequence. However, within-sample variation may be as great as the entire stratigraphic variation. Halogen contents of marginal samples generally are similar to axial samples, but tend not to have as high Cl concentration and tend to OH-enrichment. Biotite compositions approach hydroxyl end-member compositions, and apatite-biotite OH-F exchange geothermometers give an average closure temperature of 564° C. Apatite from the Umvimeela Dyke, an unlayered dike that parallels the Great Dyke over much of its length, contains less Cl than is seen in the Ultramafic Sequence cumulates of the Great Dyke. While the overall stratigraphic trend is characterized by a decrease in the Cl/F ratio with stratigraphic height, within the P1 unit at the top of the Ultramafic Sequence there is a positive correlation between Cl and other incompatible elements such as Na and Ce. The apparent contradiction between the general stratigraphic trend of decreasing Cl/F ratio with fractionation and the apparent increase in Cl and other incompatible elements seen in the P1 unit can be explained by assuming that the Great Dyke magma chamber was degassing near its top, where confining pressure was lowest and Cl was preferentially lost to a separating volatile-rich fluid. As cumulates formed on the floor, they entrapped liquid that was increasingly depleted in Cl at the higher stratigraphic levels. However, at any given stratigraphic interval, either local fluid enrichment or the eventual crystallization of halogen-bearing minerals that incorporate the smaller F ion in preference to the larger Cl ion led to a local increase in the Cl/F ratio. Received: 31 October 1994/Accepted: 13 June 1995     

Magma influx and mixing in the Bjerkreim-Sokndal layered intrusion, South Norway: evidence from the boundary between two megacyclic units at Storeknuten

The Bjerkreim-Sokndal layered intrusion belongs to the Proterozoic anorthositic province in the Rogaland area of southern Norway. The northwestern part of the intrusion comprises a ca. 6 km-thick Layered Series made up of megacyclic units (MCU) arranged in a syncline; each megacyclic unit reflects the influx of fresh magma into the chamber. The boundary between megacyclic units III and IV has been studied in detail at Storeknuten on the southern flank of the syncline. The megacyclic units can be subdivided into a series of cumulate stratigraphic zones; the interval from the top of zone IIIe to the base of zone IVd is exposed in the Storeknuten area. Modally layered plagioclase-hypersthene-ilmenite-magnetite-augite-apatite cumulates belonging to zone IIIe are overlain by 30 m of massive plagioclase-rich rocks (commonly containing ilmenite and/or hypersthene) constituting zone IVa. The entry of cumulus olivine defines the base of zone IVb (dominantly plagioclase-olivine-ilmenite cumulates) which is about 100 m thick. Many of the olivines are partly or completely replaced by Ca-poor pyroxene/Fe---Ti oxide symplectites. This massive leucotroctolitic zone is overlain by modally layered, laminated plagioclase-hypersthene-ilmenite cumulates of zone IVc. The successive entry of magnetite, apatite (accompanied by Ca-rich pyroxene) and inverted pigeonite defines zones IVd, e and f respectively. The entry of K-feldspar (accompanied by Fe-rich olivine) defines the base of a jotunitic transition zone which passes upwards into mangerites and quartz mangerites.

There is a compositional regression through zone IVa. The upper part of zone IIIe has Ca-poor pyroxene with about En68, plagioclase with An44–48 and a Sr-isotope ratio of about 0.7062, while the base of zone IVb has olivine with Fo75 together with En78, An53 and 0.7050 respectively. Similar reversals are shown by the minor element compositions of plagioclase and Fe---Ti oxides. Sr-isotope ratios increase systematically up through zone IVb (reaching 0.7058 in zone IVd) while An% and Sr in plagioclase and Ni and Cr in Fe---Ti oxides decrease. Olivine compositions vary unsystematically and are believed to have changed their Fe:Mg ratios as a result of trapped liquid shift.

The magma residing in the chamber when the influx at the base of megacyclic unit IV took place was compositionally zoned, and assimilation of gneissic country rock at the roof had resulted in the Sr-isotope ratio increasing up through the magma column. The new magma had a Sr-isotope ratio of about 0.7050 while the resident magma had a ratio of 0.7062 at the floor, increasing upwards. The new magma mixed with the basal layer(s) of the compositionally zoned resident magma and crystallization of this hybrid magma during influx and mixing produced the compositional regression in zone IVa. When magma influx ceased, olivine-bearing rocks began to crystallize at the base of zone IVb. The leucotroctolites at the base of this zone are the most primitive rocks in the entire intrusion. The systematic increase in Sr-isotope ratios up through zone IVb resulted from progressive mixing between new and resident magma. This mixing either took place during magma influx or by the progressive mixing of overlying resident magma layers during crystallization.

Calculations based on geochemical modelling, the thickness of cumulate stratigraphy repeated and Sr-isotope ratios indicate that the new magma influx had a thickness of 350–500 m in the Storeknuten section and that the leucotroctolites of zone IVb represent about 20–30% crystallization of this influx.     

Slow,dense replenishments of a basic magma chamber: the layered series of the Newark Island layered intrusion,Nain, Labrador

The Newark Island layered intrusion is a composite layered intrusion within the Nain anorthosite complex, Labrador. The intrusion comprises a lower layered series (LS) dominated by troctolites, olivine gabbros and oxide-rich cumulates and an upper hybrid series (HS) characterized by a wide range of mafic, granitic and hybrid cumulates and discontinuous layers of chilled mafic rocks (Wiebe 1988). The HS crystallized from a series of replenishments of both silicic and basic magmas. The LS crystallized from periodically replenished basic magmas. The LS has a lower zone that consists mainly of olivine-plagioclase cumulates and contains minor cryptic reversals in mineral compositions that resulted from replenishments of relatively primitive magma. An upper zone is dominated by olivine-plagioclaseaugite-ilmenite cumulates. Cumulus titanomagnetite and pyrrhotite occur within some oxide-rich cumulates, and the stratigraphically highest layers contain cumulus apatite. At intermediate levels in the sequence, cumulus inverted pigeonite occurs in place of olivine. Several prominent regressions in the stratigraphy of the upper zone are marked by fine-grained troctolitic layers with much higher Mg no. [100 MgO/(MgO+FeO)] and anorthite than underlying cumulates. These layers coarsen upward and grade back to oxide-bearing olivine gabbros within thicknesses ranging from 10 cm to 15 m. Dikes that cut the LS have major- and trace-element compositions that strongly suggest that they are feeders for the replenishments. In the lower zone when olivine and plagioclase were the only cumulus phases, replenishments were less dense than the resident magma and rose as plumes and mixed with it. Precipitation of cumulus oxides in the upper zone lowered the density of resident magma so that subsequent replenishments were more dense than resident magma. Replenishments that occurred after oxides began to precipitate had small injection velocities. These post-oxide injections flowed along the interface between resident magma and the cumulate pile and precipitated flow-banded, fine-grained troctolites.     

Investigations in the Stillwater Complex: Part II. Petrology and Petrogenesis of the Ultramafic Series

The Ultramafic series of the Stillwater Complex has been dividedinto two major zones: a Peridotite zone formed of 20 macro-rhythmicunits of dunite-harzburgite-orthopyroxenite, and an overlyingOrthopyroxenite zone. The stratigraphic section has been determinedat Mountain View (2065 m) and at Chrome Mountain (840 m). TheMountain View section apparently formed in a subsiding basinwhereas the rocks at Chrome Mountain accumulated in a relativelystable, higher area of the chamber floor. In both sections,Mg/(Mg + Fe) in cumulus mafic minerals increases with stratigraphicheight in the lower 400 m, then remains relatively constantthrough the rest of the series. The base of the series is marked by the first appearance oflaterally extensive olivine-rich cumulates. The accretion ofthe cumulates and the growth of the chamber proceeded throughperiodic injections of olivine-saturated mafic magma. The lowercontact of the cycles represents a hiatus in crystallizationand a return to a more primitive magma composition. Althoughhotter, the primitive magma was more dense, so it entered thechamber at or near the floor and did not immediately mix withthe more differentiated orthopyroxene-saturated magma alreadypresent. As it cooled by transfer of heat across its upper surface,the primitive magma crystallized olivine and differentiatedin situ to form the lower dunite. With the accumulation of olivinenear the base, the crystal/liquid ratio, and thus the density,decreased at the top of the layer eventually resulting in mixingand the formation of harzburgite. After removal of olivine byresorption and settling from the hybrid magma, orthopyroxenealone crystallized forming an orthopyroxenite. Chromitite layersprobably formed by the mixing of primitive olivine± chromite-saturatedmagma and narrow layers of orthopyroxene-saturated magma trappedunderneath. The Mg-enrichment trend in the lower 400 m resulted from reactionof cumulus olivine and/or orthopyroxene with progressively decreasingvolumes of intercumulus liquid. As heat loss through the floordecreased, accumulation rate approached a steady state, thefraction of trapped liquid remained more or less constant andvariation in Mg/(Mg + Fe) was governed dominantly by cumulusprocesses. The constant NiO abundances in olivine throughoutthe section are consistent with the model for the formationof the macro-rhythmic units. Depletion of NiO was dampened byrepeated additions of parental magma, localized equilibriumcrystallization, mixing, and the effect of postcumulus equi-librationwith varied amounts of trapped liquid. Discordant dunite bodies, which are common at Chrome Mountain,formed by the replacement by olivine of earlier formed cumulates.The replacement involved the incongruent dissolution of ortho-pyroxeneat near-solidus temperatures by a late-stage, hydrous vaporprobably derived from the magma. The vapor phase migrated alongfractures formed by the readjustment of the cumulate pile.     

The Fedorivka layered intrusion (Korosten Pluton, Ukraine): An example of highly differentiated ferrobasaltic evolution

This study documents the petrography and whole-rock major and trace element geochemistry of 38 samples mainly from a drill core through the entire Fedorivka layered intrusion (Korosten Pluton), as well as mineral compositions (microprobe analyses and separated mineral fraction analyses of plagioclase, ilmenite, magnetite and apatite) of 10 samples. The Fedorivka layered intrusion can be divided into 4 lithostratigraphic units: a Lower Zone (LZ, 72 m thick), a Main Zone (MZ, 160 m thick), and an Upper Border Zone, itself subdivided into 2 sub-zones (UBZ₂, 40 m thick; UBZ₁, 50 m thick). Igneous lamination defines the cumulate texture, but primary cumulus minerals have been affected by trapped liquid crystallization and subsolidus recrystallization. The dominant cumulus assemblage in MZ and UBZ₂ is andesine (An_39–42), iron-rich olivine (Fo_32–42), augite (En_29–35Fs_24–29Wo_42–44), ilmenite (Hem_1–6), Ti-magnetite (Usp_52–78), and apatite. The data reveal a continuous evolution from the floor of the intrusion (LZ) to the top of MZ, due to fractional crystallization, and an inverse evolution in UBZ, resulting from crystallization downwards from the roof. The whole-rock Fe/Mg ratio and incompatible element contents (e.g. Rb, Nb, Zr, REE) increase in the fractionating magma, whereas compatible elements (e.g. V, Cr) steadily decrease. The intercumulus melt remained trapped in the UBZ cumulates due to rapid cooling and lack of compaction, and cumulus mineral compositions re-equilibrated (e.g. olivine, Fe–Ti oxides). In LZ, the intercumulus melt was able to partially or totally escape. The major element composition of the MZ cumulates can be approximated by a mixing (linear) relationship between a plagioclase pole and a mafic pole, the latter being made up of all mafic minerals in (nearly) constant relative proportions. By analogy with the ferrobasaltic/jotunitic liquid line of descent, defined in Rogaland, S. Norway, and its conjugated cumulates occurring in the Transition Zone of the Bjerkreim-Sokndal intrusion (Rogaland, a monzonitic (57% SiO₂) melt is inferred to be in equilibrium with the MZ cumulates. The conjugated cumulate composition falls (within error) on the locus of cotectic compositions fixed by the 2-pole linear relationship. Ulvöspinel is the only Ti phase in some magnetites that have been protected from oxidation. QUIlF equilibria in these samples show that magnetite and olivine in MZ have retained their liquidus compositions during subsolidus cooling. This permits calculation of liquidus fO₂ conditions, which vary during fractionation from ΔFMQ = 0.7 to − 1.4 log units. Low fO₂ values are also evidenced by the late appearance of cumulus magnetite (Fo₄₂) and the high V³⁺-content of the melt, reflected in the high V-content of the first liquidus magnetite (up to 1.85% V).     

Fractional crystallization and magma mixing in the Tigalak layered intrusion,the Nain anorthosite complex,Labrador

The Tigalak intrusion is a dominantly dioritic layered body, about 80 km² in area, which ranges in composition from norite to granodiorite. Local areas of the layered rocks display upward fractionation from norite to ferrodiorite. Periodic reversals of mineral composition trends record the emplacement of less fractionated dioritic magma. Heterogeneous mixtures of dioritic and granodioritic rocks occur widely in mappable lenses and layers that alternate up section and along the strike with more uniformly layered rocks. In these mixtures, chilled dioritic pillows occur abundantly in a hybrid cumulate matrix of granodiorite to diorite composition. Cross-cutting granodioritic dikes grade upward into stratigraphically-bound lensoid masses of the hybrid cumulates. It appears that the hybrid rocks formed as a result of the emplacement of the granodioritic magma through lower cumulates into the dioritic magma chamber and that the dioritic pillows represent chilled bodies of Ferich dioritic magma that commingled with cooler granodioritic magma and settled to the floor of the Tigalak magma chamber. The restricted distribution of these mixtures of hybrid cumulates and chilled pillows indicates that mixing between granodioritic and dioritic liquids was limited in time and lateral extent. Periodic injections of granodioritic liquids may have collected as a separate layer below the roof of the magma chamber and above dioritic magma.     

The Mordor Alkaline Igneous Complex,Central Australia: PGE-enriched disseminated sulfide layers in cumulates from a lamprophyric magma

The Mordor Alkaline Igneous Complex (MAIC) is a composite intrusion comprising a body of syenite and a funnel-shaped layered mafic–ultramafic intrusion of lamprophyric parentage, the Mordor Mafic–Ultramafic Intrusion or MMUI. The MMUI is highly unusual among intrusions of lamprophyric or potassic parentage in containing primary magmatic platinum-group element (PGE)-enriched sulfides. The MMUI sequence consists largely of phlogopite-rich pyroxenitic cumulates, with an inward dipping conformable layer of olivine-bearing cumulates divisible into a number of cyclic units. Stratiform-disseminated sulfide accumulations are of two types: disseminated layers at the base of cyclic units, with relatively high PGE tenors; and patchy PGE-poor disseminations within magnetite-bearing upper parts of cyclic units. Sulfide-enriched layers at cycle bases contain anomalous platinum group element contents with grades up to 1.5 g/t Pt+Pd+Au over 1-m intervals, returning to background values of low parts per billion (ppb) on a meter scale. They correspond to reversals in normal fractionation trends and are interpreted as the result of new magma influxes into a continuously replenished magma chamber. Basal layers have decoupled Cu and PGE peaks reflecting increasing PGE tenors up-section, due to increasing R factors during the replenishment episode, or progressive mixing of between resident PGE-poor magma and more PGE-enriched replenishing magma. The presence of PGE enriched sulfides in cumulates from a lamprophyric magma implies that low-degree partial melts do not necessarily leave sulfides and PGEs in the mantle restite during partial melting. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Magma Mixing and Intraplutonic Quenching in the Wingellina Hills Intrusion, Giles Complex, Central Australia

The Wingellina Hills intrusion is a small composite gabbroic/ultramaficintrusion and forms a tectonically dismembered segment of theUpper Proterozoic Giles complex in central Australia. Its 1600m of exposed magmatic stratigraphy formed in a continuouslyfractionating, periodically replenished magma chamber. Olivinegabbro and gabbronorite units alternate with lenticular strataboundintercalations of ultramafic (peridotite and pyroxenite) cumulates.A well-developed hybrid footwall zone of intermingled gabbroand pyroxenite underlies each ultramafic unit and demonstratesthe intrusive relationships of ultramafics into gabbroic cumulatemembers. The limited range of mg-number [100 ? Mg/(Mg+Fe)] of ferromagnesiansilicates indicates that the magmatic sequence covers a rathersmall spectrum in chemical fractionation and that the WingellinaHills intrusion represents the basal portion of a formerly largerlayered complex. The mg-number of olivine ranges from 89 to77, below which olivine is replaced by cumulus orthopyroxene.Clinopyroxene covers a wider mg-number range from 91 to 77 andis systematically enriched in MgO relative to coexisting orthopyroxeneand olivine. Anorthite content in plagioclase generally correlatespositively with mg-number changes of coexisting ferromagnesiansilicates. Interstitial plagioclase in clinopyroxenites containsexsolution lamellae of pure orthoclase. These antiperthitesare among the most calcic recorded, with plagioclase hosts betweenAn₆₀ and An₈₀. Bulk antiperthite compositions range around An₆₅–Ab₁₅–Or₂₀and straddle a high-temperature (Or₂₀) solvus in the plagioclasetriangle. The extent of former solid solution between calcicplagioclase and orthoclase indicates crystallization and coolingof the cumulates under moderate pressure and anhydrous conditions. Cryptic mg-number variations show that the intrusion experiencedweak iron enrichment with stratigraphic height. Normal fractionationis confined to the gabbroic members of the sequence, whereasultramafic intercalations are associated with sharp chemicalreversals toward more refractory mineral compositions. Reversalsof mg-number are considerably displaced into the underlyinggabbroic units by up to 50 m relative to the basis of ultramaficintercalations, which indicates extensive postcumulus infiltrationmetasomatism following the emplacement of fresh magma. The trivalentoxides in clinopyroxene have retained their pristine stratigraphicvariation patterns through later metasomatic events and stillcoincide with the cumulus layering. Macroscopic and cryptic layering in the Wingellina Hills intrusionare consistent with a continuously fractionating magma chamberwhose differentiation path was repeatedly reset by periodicinfluxes of primitive parent melt. Ultramafic and gabbroic cumulatemembers can be derived from a single olivine-saturated parentmelt by sequential separation of olivine, olivine-clinopyroxene,and finally olivine/orthopyroxene-clinopyroxene-plagioclase.A series of orthopyroxene-rich cumulates in the mixing zonesof the two melts crystallized from hybrids of the most primitiveand most evolved end-member compositions. Liquidus temperatures calculated for the resident and replenishingmelt components yield 1250 and 1350?C, respectively. As a resultof this temperature difference, fresh influxes of hot parentliquid crystallized rapidly under strongly undercooled conditionsas they ponded on, and quenched against,the chamber floor. Rapidcooling caused a temporary acceleration of the crystallizationfront and formation of impure cumulates with high trapped meltproportions, which resulted in a close coincidence of orthocumulateunits with stratigraphic levels of primitive melt addition.Grain sizes in orthocumulates vary with the cooling rate andpass through a maximum as the degree of undercooling increases.High cooling rates also influenced the composition of some cumulusphases. Clinopyroxenes from ultramafics in the mixing zonesare enriched in iron and aluminium (despite a more primitiveparent melt) and fall outside the fractionation path, especiallyif the batch of new hot magma was small compared with the poolof cooler resident liquid. Aluminous cumulus spinel is partof a metastable crystallization sequence and only crystallizedin the most magnesian ultramafics after episodes of intraplutonicquenching.     

The Geology of the Great Dyke, Zimbabwe: Crystallization, Layering, and Cumulate Formation in the P1 Pyroxenite of Cyclic Unit 1 of the Darwendale Subchamber

The P1 layer of the Great Dyke is an 200 m thick pyroxenitesuccession in Cyclic Unit 1 and, as the topmost lithology ofthe Ultramafic Sequence, represents the transition from ultramaficto mafic rocks. Of critical importance to this part of the stratigraphyis the strong lateral environmental change from axis to marginas a result of the flared structure of the Great Dyke. Duringthe formation of the P1 layer the axial zone was underlain bya great thickness of hot ultramafic cumulates whereas the samelayer in the marginal zone progressively offiaps the lower ultramaficlayers and is in close proximity to the underlying wall/floorrocks. Heat loss through the floor was therefore much greaterin the marginal zone than in the axis. Major lateral variations are observed, with all lithologicalunits and layers thinning towards the margins of the subchambertogether with a progressive change in the form of the cumulates.Discordant relationships towards the margin between layer types(modal, cryptic, and form) are a feature of the P1 unit whichhas also been recognized in other parts of the Great Dyke (Prendergast,1991). Pyroxene compositions show significant variations withinan overall fractionation trend and decoupling occurs betweenmajor and minor element components of bronzite, suggesting strongcompositional heterogeneity of the magma. This type of crypticlayering has not previously been described and is informallycalled ‘cryptorhythmic’ layering. Pyroxene compositional variation is related to reaction andmodification by trapped intercumulus liquid, and few mineralspreserve liquidus compositions. A similar situation must existfor most layered intrusions. The strong dependence of pyroxenecompositions on incompatible element content in the whole-rockshows that the original liquidus compositions were modifiedby postcumu-lus overgrowth and reaction with the trapped intercumulusliquid. Well-constrained data arrays indicate that most cumulatesin the P1 layer behaved as a closed system with little or nomigration of intercumulus liquid. Liquidus compositions cantherefore be deduced and the residual porosity and degree ofpostcumulus formation were modelled using a computer program.Residual porosity is shown to be between 1 and 13% (by mass).Rocks in the marginal facies have a relatively large proportionof discrete postcumulus phases but instead of representing crystallizationof trapped liquid these are shown to be mainly heteradcumulusphases, i. e., interstitial minerals that have grown largelyby adcumulus processes in equilibrium with the main body ofmagma. The heteradcumulus component can be as high as 27%. Thesephases occur as oikocrysts which give rise to a well-developednodular pyroxenite (the ‘potato’ reef). The formationof the nodules caused local redistribution of primary sulphideliquid. The liquid layers which gave rise to cumulates in the marginalfacies are shown to be enriched in iron and incompatible elementscompared with the axial zone, indicating that the P1 pyroxenitelayer formed by crystallization of a magma which was eithercompositionally stratified or exhibited a strong lateral compositionalgradient.     

The Salt Lick Creek layered intrusion,East Kimberley region,Western Australia

The Lower Proterozoic Salt Lick Creek intrusion, East Kimberley region, Western Australia, is a layered intrusion divisible into two well-defined zones, the Basal and Main Zones, whose combined stratigraphic thickness, as now exposed, is approximately 1000 metres. The Basal Zone, 360 metres thick, contains three members, two of which (Members 1 and 3) are dominated by olivine, plagioclase cumulates (including harrisites and allivalites); Member 2, near the middle of the Basal Zone, consists substantially of more olivine-rich cumulates, including plagioclase-bearing dunites. The Main Zone, commencing with Member 4 plagioclase, orthopyroxene cumulates, is composed largely of anorthositic cumulates of Member 5. Mild but nevertheless measurable rhythmic layering is superimposed upon the three members comprising the Basal Zone. Electron probe microanalyses of the primary phases across some 500 metres of cumulates indicate limited cryptic variation with stratigraphic height. Olivine ranges in composition from Fo₈₁ to Fo₈₄, orthopyroxene from Ca₂Mg₈₃Fe₁₅ to Ca₂Mg₇₈Fe₂₀, clinopyroxene from Ca₄₈Mg₄₆Fe₆ to Ca₄₄Mg₄₈Fe₈, and plagioclase from An₈₄ to An₈₈ but mineral compositions are not a simple function of stratigraphic height. It is inferred that the parental magma(s) was high-alumina mafic, intrinsically subalkaline, strongly olivine- and plagioclase-normative and in all likelihood tholeiitic in its affinities. The olivine-free cumulates of the Main Zone display a higher level of normative saturation than the cumulates of the Basal Zone but mineral and host rock chemistries, particularly 100 Mg/ (Mg+Fe²⁺) atomic ratios, are not favourable to proposals which would relate the origin of the Main Zone or the several members of the intrusion to the differentiation of a single pool of magma. It is suggested that the Main Zone, at least, derived from a separate pulse of relatively more saturated magma and that the lateral replenishment by more or less undifferentiated magma was also a fundamental and critical factor in the genesis of the Basal Zone cumulates.     

Some problems with the cumulus theory

There are a number of features in layered intrusions which do not appear to be consistent with the cumulus theory. The most important of these are: (1) Rhythmic layering in plagioclase-pyroxene cumulates from some intrusions (e.g. Jimberlana and Stillwater) may be inverted with the light plagioclase-rich cumulates at the base of the layer and heavy pyroxene-rich cumulates at the top. (2) Hydraulic sorting in rhythmically layered cumulates is often poor, suggesting that chemical and not mechanical processes control the distribution of minerals in this type of layered sequence. (3) The textures of cumulates from the steeply dipping marginal zone of Jimberlana, which cannot have formed by gravity settling, are indistinguishable from those found in the flatly dipping central layers of the intrusion. Similarly, it is difficult to explain the cumulate textures in the ‘overturned’ marginal layered series of Jimberlana. (4) Igneous layering, including cross-bedding, graded bedding and trough banding, has been described in layered sequences with near-vertical primary dips. (5) Calculations and experiments with a centrifuge furnace suggest that plagioclase cannot sink in Fe-rich tholeiitic liquids. (6) The settling of crystals as individual grains assumes that nucleation is homogeneous. This is unlikely.

It is probable that heterogeneous and self-nucleation are the dominant nucleation mechanisms during the formation of cumulates. This gives rise to two possibilities. Firstly, that most grains nucleate against pre-existing settling crystals to form composite grains which gravitate to the floor of the chamber. Secondly, that the cumulus grains nucleate in situ at the temporary floor of the magma chamber.     

Petrology of the Marginal Border Series of the Skaergaard Intrusion

The Marginal Border Series (MBS) of the Skaergaard intrusionconsists of rocks formed by in situ crystallization againstthe walls of the intrusion. Most of these rocks are productsof fractional crystallization, though samples believed to representchilled liquid occur locally at the intrusive contact. The MBScomprises only 5% of the exposed volume of the intrusion, butwithin its thickness, the order of crystallization and the compositionsof fractionated rocks and minerals vary systematically withdistance inward from the intrusive contact in largely the samemanner as rocks and minerals upward through the Layered Series(LS). Earliest differentiates are cumulates of olivine and plagioclase.The most basic compositions of cumulus plagioclase (An₇₂) andolivine (Fo₈₄) in these rocks indicate that the amount of fractionationpreceding formation of the exposed LS was substantially lessthat previously believed. Field and compositional data indicatethat picritic blocks are xenoliths rather than cumulates ofthe Skaergaard magma. Xenoliths of gneiss in all stages of reactionare locally abundant; however, there is no evidence that uppercrustal material contaminated the magma from which the MBS cumulatesformed. Compositions of cumulus minerals in the MBS differ fromthose in comparable LS rocks. Cumulates in the lower marginscontain more calcic plagioclase, more magnesian augite in allbut the late differentiates, and more iron-rich olivine. Thecompositions of cumulus olivine and to a lesser degree thoseof other mafic silicates, were modified to more iron-rich compositionsby re-equilibration with relatively large amounts of interstitialliquid. The lower MBS and LS crystallized from the same magma, but fractionationoccurred at different rates on the walls and floor of the intrusion.The upper margin may have crystallized from a magma of modifiedcomposition and fractionated at rates different from that inthe lower margin and Upper Border Series (UBS). Crystals onthe floor and roof of the intrusion accumulated faster or moreefficiently than on the walls. At any given stage of fractionation,crystals also accumulated against all sides of the magma chamberat about the same rate. Either the rates of cooling, crystallization,and crystal retention affected accumulation rates locally asfunctions of rock type and geometry of the walls, or these rateswere largely independent of wall rock owing to buffering ofconductive heat loss possibly to an envelope of hydrothermalfluid circulating around the crystallizing magma. The appearanceor disappearance of cumulus minerals in the lower MBS occursat higher structural levels than in the LS and at lower structurallevels than in the UBS. These relationships together with cumulusmineral compositions indicate that magma at the margins wasalways somewhat less fractionated than that at the floor androof of the chamber. It is proposed that these relationshipsreflect the combined effects of liquid and crystal fractionationof the magma within largely independent convection systems inthe lower and upper parts of the chamber.     

The petrology of the layered gabbro intrusion, eastern gabbro, Coldwell alkaline complex, Northwestern Ontario, Canada: evidence for multiple phases of intrusion in a ring dyke

The Coldwell alkaline complex is a large (> 350 km²) gabbro and syenite intrusion on the north shore of Lake Superior. It was emplaced at 1108 Ma during early magmatic activity associated with the formation of the Mid-Continent Rift of North America. The eastern gabbro forms a partial ring dyke on the outer margin of the complex and consists of at least three discrete intrusions. The largest of these is the layered gabbro that comprises a 300 m thick fine- to medium-grained basal unit overlain by up to 1100 m of variably massive to layered gabbroic cumulates which vary from olivine gabbro to anorthosite. Several xenoliths of Archaean metamorphic rocks that range in size from 10's to 100's of meters are present in the central part of the intrusion. Within discrete horizons in the layered gabbro are many centimeter- to meter-scale, gabbroic xenoliths. The main cumulus minerals, in order of crystallization, are plagioclase, olivine and clinopyroxene ± Fe-Ti oxides. Biotite and Fe-Ti-oxide are the dominant intercumulus phases. Orthopyroxene occurs not as a cumulus phase but as peritectic overgrowths on cumulus olivine. A detailed petrographic and mineral chemical study of samples from two stratigraphically controlled traverses through the layered gabbro indicates that the stratigraphy cannot be correlated along the 33 km strike of the ring dyke. Mineral compositions show both normal and reversed fractionation trends. These patterns are interpreted to record at least three separate intrusions of magma into restricted dilatant zones within the ring dyke possibly associated with ongoing caldera collapse. Calculations of parental melt composition using mineral — melt equilibria show that even the most primitive gabbros crystallized from an evolved magma with mg# of 0.42-0.49. The presence of orthopyroxene overgrowths on cumulus olivine suggests rising silica activity in the melt during crystallization and implies a subalkaline parentage for the layered gabbro.     

Pyroxenes of the Dufek Intrusion, Antarctica

The Dufek intrusion is a stratiform mafic body, 24,000 to 34,000km² in area and 8 to 9 km thick, in the Pensacola Mountainsof Antarctica. Textures, structures, magmatic stratigraphy,and chemical variation indicate that layered gabbros and relatedrocks of this body developed by accumulation of crystals thatsettled on the floor of a magma chamber. The major cumulus phasesin the exposed part of the intrusion are plagioclase, pyroxene,and iron-titanium oxides. The base of the Dufek intrusion is not exposed, and both Ca-richand Ca-poor pyroxene coexist as cumulus phases in the lowerexposed rocks. The Ca-rich pyroxenes belong to an augite-ferroaugiteseries (Ca_36.4Mg_48.7Fe_14.9-Ca_30.0Mg_23.5Fe_46.5) that extendsup through the 300 m thick capping granophyre. The Ca-poor pyroxenesbelong to a bronzite-inverted pigeonite series (Ca_3.5Mg_69.1Fe_27.4-Ca_11.4Mg_34.0Fe_54.6)that extends only to about 200 m below the granophyre layer.In addition to the cumulus pyroxenes some rocks contain post-cumulusgreen calcic augite and ferrohypersthene. The compositional change of the cumulus pyroxenes with stratigraphicheight is one of general iron enrichment. Superimposed on thistrend are (1) a 1 km thick section in the lower part of thebody that shows slight to no iron enrichment and (2) a markedreversal in the Fe/(Fe+Mg) ratio about 1 km below the top ofthe body. The variations from the general trend are associatedwith cyclic units and are best explained by convective overturnof the magma. In general, the pyroxene compositional trends are similar tothose of the Skaergaard and Bushveld intrusions. One significantdifference in the Dufek intrusion is the limited iron enrichmentof its Ca-rich pyroxenes, that may relate to a slower decreaseof P_O2 during crystallization of the Dufek magma.     

Re-Os and Sm-Nd Isotope and Trace Element Constraints on the Origin of the Chromite Deposit of the Ipueira-Medrado Sill, Bahia, Brazil

The chromite deposit of the Paleoproterozoic Ipueira–Medradosill is hosted in a single, thick (5–8 m), massive layer,which sets severe constraints for the origin of chromitites.It is divided from bottom to top into: (1) a Marginal Zone (5–20m); (2) an Ultramafic Zone (<250 m) consisting of duniteand harzburgite that host the chromitite layer, in which intercumulusamphibole is important and more abundant toward the top; (3)a Mafic Zone (<40 m). The parental magma was large ion lithophileelement and light rare earth element enriched and high fieldstrength element depleted. Sm–Nd isotopic compositionsare consistent with a 2 Ga age, but suggest a variable initialNd isotopic composition that correlates with the abundance ofamphibole. The more negative     

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway) Part II. REE and the trapped liquid fraction

Rare earth elements in bulk cumulates and in separated minerals (plagioclase, apatite, Ca-poor and Ca-rich pyroxenes, ilmenite and magnetite) from the Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway) are investigated to better define the proportion of trapped liquid and its influence on bulk cumulate composition. In leuconoritic rocks (made up of plagioclase, Ca-poor pyroxene, ilmenite, ±magnetite, ±olivine), where apatite is an intercumulus phase, even a small fraction of trapped liquid significantly affects the REE pattern of the bulk cumulate, together with cumulus minerals proportion and composition. Contrastingly, in gabbronoritic cumulates characterized by the presence of cumulus Ca-rich pyroxene and apatite, cumulus apatite buffers the REE content. La/Sm and Eu/Eu* vs. P₂O₅ variations in leuconorites display mixing trends between a pure adcumulate and the composition of the trapped liquid, assumed to be similar to the parental magma. Assessment of the trapped liquid fraction in leuconorites ranges from 2 to 25% and is systematically higher in the north-eastern part of the intrusion. The likely reason for this wide range of TLF is different cooling rates in different parts of the intrusion depending on the distance to the gneissic margins. The REE patterns of liquids in equilibrium with primitive cumulates are calculated with mass balance equations. Major elements modelling (Duchesne, J.C., Charlier, B., 2005. Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway): Part I. Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent. Lithos. 83, 299–254) permits calculation of the REE content of melt in equilibrium with gabbronorites. Partition coefficients for REE between cumulus minerals and a jotunitic liquid are then calculated. Calculated liquids from the most primitive cumulates are similar to a primitive jotunite representing the parental magma of the intrusion, taking into account the trapped liquid fraction calculated from the P₂O₅ content. Consistent results demonstrate the reliability of liquid compositions calculated from bulk cumulates and confirm the hypothesis that the trapped liquid has crystallized as a closed-system without subsequent mobility of REE in a migrating interstitial liquid.     

Character and significance of spectacular layering features developed in the thin, alkali-basaltic sills of the Ulvö Gabbro Complex, Sweden

Summary The Ulv? Gabbro Complex consists of alkali-olivine basaltic circular bodies ∼30–80 km in diameter. These intrusions were emplaced at shallow depths (∼3 km) as thin sheets (∼300 m). Among other things, the gabbroic cumulates of the complex display: modal layering, grain-size variations, trough structures, and slump structures. The crystallization sequence is olivine+plagioclase, ulv?spinel, clinopyroxene, and apatite. A nearly continuous exposure across one of these intrusions, the Norra Ulv?n gabbro, is subdivided into: a Lower Zone (LZ), a Rhythmically Layered Zone (RZ) and an Upper Zone (UZ). LZ and RZ were formed at the floor, while UZ grew from the roof downward. Major-element variations in the cores of the cumulus minerals define fractionation trends from the base of the intrusion to the RZ-UZ boundary interpreted as a “sandwich horizon”. Modeling suggests that a significant amount of crystallized interstitial liquid is required to produce the observed stratigraphic relations. Our results suggest that the small size and shallow emplacement depth of the intrusions of the Ulv? Gabbro Complex helped to preserve evidence of primary accumulation processes. However, it is also clear that despite the limited time available postcumulus processes such as diffusional homogenization and compaction of some grains were important. Correspondence: S. ?. Larson, Earth Sciences Centre, Department of Geology, G?teborg University, POB 460, SE 40530 G?teborg, Sweden     

Chromitites from the Luanga Complex,Carajás,Brazil: Stratigraphic distribution and clues to processes leading to post-magmatic alteration

The Neoarchean (ca. 2.75 Ga) Luanga Complex, located in the Carajás Mineral Province in Brazil, is a medium-size layered intrusion consisting, from base to top, of ultramafic cumulates (Ultramafic Zone), interlayered ultramafic and mafic cumulates (Transition Zone) and mafic cumulates (Mafic Zone). Chromitite layers in the Luanga Complex occur in the upper portion of interlayered harzburgite and orthopyroxenite of the Transition Zone and associated with the lowermost norites of the Mafic Zone. The stratigraphic interval that hosts chromitites (∼150 meters thick) consists of several cyclic units interpreted as the result of successive influxes of primitive parental magma. The compositions of chromite in chromitites from the Transition Zone (Lower Group Chromitites) have distinctively higher Cr# (100Cr/(Cr + Al + Fe³⁺)) compared with chromite in chromitites from the Mafic Zone (Upper Group Chromitites). Chromitites hosted by noritic rocks are preceded by a thin layer of harzburgite located 15–20 cm below each chromitite layer. Lower Cr# in chromitites hosted by noritic rocks are interpreted as the result of increased Al₂O₃ activity caused by new magma influxes. Electron microprobe analyses on line transverses through 35 chromite crystals indicate that they are rimmed and/or extensively zoned. The composition of chromite in chromitites changes abruptly in the outer rim, becoming enriched in Fe³⁺ and Fe²⁺ at the expense of Mg, Cr, Al, thus moving toward the magnetite apex on the spinel prism. This outer rim, characterized by higher reflectance, is probably related to the metamorphic replacement of the primary mineralogy of the Luanga Complex. Zoned chromite crystals indicate an extensive exchange between divalent (Mg, Fe²⁺) cations and minor to none exchange between trivalent cations (Cr³⁺, Al³⁺ and Fe³⁺). This Mg-Fe zoning is interpreted as the result of subsolidus exchange of Fe²⁺ and Mg between chromite and coexisting silicates during slow cooling of the intrusion. A remarkable feature of chromitites from Luanga Complex is the occurrence of abundant silicate inclusions within chromite crystals. These inclusions show an adjacent inner rim with higher Cr# and lower Mg# (100 Mg/(Mg + Fe²⁺)) and Al# (100Al/(Cr + Al + Fe³⁺)). This compositional shift is possibly due to crystallization from a progressively more fractionated liquid trapped in the chromite crystal. Significant modification of primary cumulus composition of chromite, as indicated in our study for the Luanga Complex, is likely to be common in non-massive chromitites and the rule for disseminated chromites in mafic intrusions.     

Snowflake troctolite in the Hettasch intrusion,Labrador: Evidence for magma-mixing and supercooling in a plutonic environment

The snowflake troctolite (SFT) in the Hettasch intrusion is a thin (0–10 m) zone of melatroctolite concordant with the normal leucotroctolites on one limb of the Hettasch intrusion. The textures of this unit are strikingly different from the normal cumulate textures of the Hettasch leucotroctolites and include comb-layered plagioclase, skeletal megacrysts of plagioclase, and spherulitic plagioclase 5–15 cm in diameter (snowflakes). These supersaturation plagioclase morphologies are set in a matrix of fine-grained, layered and unlayered melatroctolite. Because the mineral compositions are more primitive in the SFT than in the surrounding Hettasch cumulates, crystallization models involving supersaturation of the basaltic Hettasch magma fail. However, over a short distance below the texturally-defined SFT and a shorter distance above it, the mineral compositions of the surrounding cumulates gradually merge with those of the SFT. It is concluded, therefore, that the SFT formed from a separate magma that irrupted onto the floor of the Hettasch magma chamber and was supercooled by the basaltic Hettasch magma. Thus the SFT magma is inferred to have been more primitive than basalt. Assuming that fractionation in the SFT was minimal or that no fractionated material has escaped the SFT, an estimate of the bulk composition of the unit should approximate the magma composition. A picritic magma is thereby inferred, and a search for comparable rocks or magma types indicates that these rocks have the most similarities with picritic rocks of the Brito-Arctic province which formed during the early opening of the North Alantic. This similarity supports the hypothesis that the anorthositic Nain complex in Labrador also formed in a rifting environment about 1.4 Gyr ago.